Along with higher performances achieved in electronic apparatuses, semiconductor elements with a higher density and highly packaged semiconductor elements have been developed. In response to this trend, it becomes essential to more efficiently radiate heat generated from electronic parts forming electronic apparatuses. In order to allow the semiconductor to radiate heat efficiently, the semiconductor is attached to a heat sink, such as a heat radiating fin, a heat radiating plate, or the like, with a thermally conductive sheet interpolated therebetween. As the thermally conductive sheet, such a sheet that is made of silicone with a filler (thermally conductive filler), such as an inorganic filler, contained and dispersed therein has been widely used.
In this heat radiating member, further improvement of a thermal conductivity is demanded, and in general, a filling rate of inorganic filler blended in a matrix is increased so as to achieve a high thermal conductivity. However, when the filling rate of the inorganic filler is increased, the flexibility is lowered, or spilled powder might occur because of the high filling rate of the inorganic filler, this method for increasing the filling rate of an inorganic filler has a limitation.
As the above-mentioned inorganic filler, for example, alumina, aluminum nitride, aluminum hydroxide, etc. are proposed. Moreover, in some cases, in order to obtain a higher heat conductivity, boron nitride (BN), scale-shaped particles, such as graphite, carbon fibers, etc. are filled into a matrix. In this case, the anisotropic property of thermal conductivity possessed by the scale-shaped particles, etc. is utilized. For example, carbon fibers have a thermal conductivity of about 600 W/m·K to 1200 W/m·K in the fiber direction. It has been known that the anisotropic property of boron nitride is such that a thermal conductivity of about 110 W/m·K is exerted in the plane direction and a thermal conductivity of about 2 W/m·K is exerted in a direction perpendicular to the plane direction.
In this manner, the plane direction of carbon fibers or scale-shaped particles is made to be the same as the thickness direction of the sheet that is a heat transmitting direction. That is, by orienting the carbon fibers or the scale-shaped particles in the thickness direction of the sheet, it becomes possible to remarkably improve the thermal conductivity. However, in the case when, after having been molded, a cured object having been subjected to a curing process is sliced into a desired thickness, since the cured object having flexibility is sliced while being deformed, concave/convex portions on the sheet surface become greater, with air being involved into the concave/convex portions, resulting in a problem of failing to effectively utilize its superior thermal conductivity.
In order to solve the above-mentioned problem, for example, Patent Literature 1 has proposed a thermally conductive rubber sheet that is formed by being punched out with blades that are aligned in a direction perpendicular to the sheet longitudinal direction with equal intervals, and then sliced. Moreover, Patent Literature 2 has proposed a method in which a laminated member, formed by stacking layers while repeatedly carrying out coating and curing processes, is sliced with a cutting device with a round rotary blade so that a thermally conductive sheet with a predetermined thickness is obtained. Furthermore, Patent Literature 3 has proposed a method in which a laminated member, formed by stacking two or more graphite layers containing anisotropic graphite particles, is cut with a metal saw so as to be oriented with 0° relative to the thickness direction of the sheet so as to obtain an expanded graphite sheet (with an angle of 90° relative to the stacked layer surface).
In these proposed cutting methods, however, since the surface roughness on a cut surface becomes higher, a greater thermal resistance is caused on the interface, resulting in a problem of a reduction in thermal conductivity in the thickness direction.
Therefore, under these circumstances, there have been strong demands for providing a thermally conductive sheet that has a reduced thermal resistance because of its small surface roughness on a cut surface with a high thermal conductivity in the thickness direction, and is suitable for use as a member to be interposed between any of various heat sources (for example, various devices, such as a CPU, a transistor, and an LED) and a heat radiating member, and a method of producing such a thermally conductive sheet.